Conventionally, there has been provided a cryo-refrigerator employing a magnetic regenerative material as shown in FIG. 7.
This cryo-refrigerator comprises a first displacer 3 which has a first chamber with an unshown regenerative material accommodated therein and which is reciprocatably fitted in a first cylinder 1, and a second displacer 7 which has a second chamber communicating with the first chamber and accommodating an unshown regenerative material and which is reciprocatably fitted in a second cylinder 5. Then, the first chamber of the first displacer 3 is switchedly communicated with a high-pressure chamber 12 having an inlet 11 or with a low-pressure chamber 14 having an outlet 13, via a valve stem 9 and a valve 10.
The communicating path from the first chamber to either the high-pressure chamber 12 or the low-pressure chamber 14 is switched over by rotating the valve 10 with a synchronous motor 15.
The cryo-refrigerator having the above constitution operates as follows.
Referring to FIG. 7, a high-pressure refrigerant gas fed from a compressor (not shown) or the like is introduced into the first chamber of the first displacer 3 from the inlet 11 via the valve 10 and the valve stem 9, where the refrigerant gas undergoes heat exchange with the regenerative material within the first chamber, thus being cooled (first stage). The refrigerant gas cooled in this way is further introduced into the second chamber within the second displacer 7, where the refrigerant gas undergoes heat exchange with the regenerative material within the second chamber, thus being further cooled (second stage).
After these processes, the valve 10 is rotated by the synchronous motor 15, so that the first chamber is communicated with the low-pressure chamber 14. Then, the high-pressure refrigerant gas that has been introduced in the first chamber and the second chamber is expanded at a breath, lowering in gas temperature. In this way, the low temperature obtained by the expansion of the refrigerant gas is kept by the regenerative material.
As described above, a cryogenic temperature is obtained by iterating the introduction of the high-pressure refrigerant gas into the first chamber and the second chamber as well as its expansion (i.e., by iterating the refrigerating cycle).
Between the second displacer 7 side and the valve stem 9 side in the first cylinder 1, there occurs a pressure difference during the refrigerating cycle. Likewise, a pressure difference occurs between the terminal end side and the first displacer 3 side in the second cylinder 5 while the refrigerating cycle is performed. Therefore, a circular seal ring 21 is attached at a step portion 20 provided in an upper outer circumferential surface of the first displacer 3, thereby sealing between the inner circumferential surface of the first cylinder 1 and the outer circumferential surface of the first displacer 3. Likewise, a circular seal ring 23 is attached at a step portion 22 provided in an upper outer circumferential surface of the second displacer 7, thereby sealing between the inner circumferential surface of the second cylinder 5 and the outer circumferential surface of the second displacer 7.
FIG. 8 is a detailed sectional view of the seal ring 21. The seal ring 21 comprises a seal member 26 formed into a U shape in cross section and having an annular groove 25 opened upward, and an annular coil spring 27 fitted into the annular groove 25 of the seal member 26. Then, an outer circumferential wall 26a of the seal member 26 is biased outward by the coil spring 27, so that the sealing portion of the outer circumferential wall 26a is put into close contact with an inner circumferential surface 1a of the first cylinder 1, by which the spacing between the first cylinder 1 and the first displacer 3 is sealed.
In this arrangement, the outer circumferential wall 26a of the seal member 26 is convexly bent radially outward with an upper portion 28 bent inward so that the spring 27 fitted in the annular groove 25 will not leap out. Thus, the sealing portion is formed by a radially outer end of the bend of the outer circumferential wall 26a.
In addition, the seal ring 23 has a similar arrangement.
However, the conventional seal ring 21 as described above has the following problem.
That is, when the temperature in the first cylinder 1 comes to a cryogenic temperature through iterations of the refrigerating cycle as described above, the seal member 26 is exposed to an atmosphere of cryogenic temperature such that the outer circumferential wall 26a undergoes heat shrinkage. Then, the outermost diameter of the seal member 26 (i.e., outer diameter of the sealing portion) becomes so small that flow-by occurs, with the result of lowered refrigeratability.
It is therefore an object of the present invention to provide a cryo-refrigerator having a seal ring which suppresses the occurrence of flow-by in the cryogenic state to thereby prevent deterioration of the refrigeratability.